The demand and development of power factor correction circuitry has been fuelled by concern over the massive use of electronic power conversion, i.e. AC to DC power supplies, and the resulting potential for contamination of AC power lines.
Power factor is defined as the ratio of the actual power (Watts) developed by an AC power system to the apparent power (i.e. volt-amperes).
Most electronic ballast and switching power supplies use a bridge rectifier and a bulk storage capacitor to derive raw DC voltage from the AC power line. Such a circuit draws power from the AC line when the instantaneous AC voltage exceeds the capacitor voltage. This occurs near the line voltage peak and results in a high charge current spike. Since power is only taken near the line voltage peaks, the resulting spikes of current are extremely non-sinusoidal and have a high harmonic content. The result is a power supply with a poor power factor where the apparent input power is much higher than the real power. The power factor (i.e. ratio of actual power developed to the apparent power) is typically in the range 0.5 to 0.7. In response, International Standards are being established to control this type of harmonic loading on power systems, for example, IEC Standard 555.2 defines the maximum levels of harmonic content a device can draw from an AC power line.
To achieve a high power factor, e.g. in the range of 0.99, the current which is drawn should have a sinusoidal wave shape and the sinusoidal current should not be more than a few degrees out of phase with the sinusoidal waveform for an AC voltage supply. Power factor correction circuits according to the art fall into two broad groups: passive and active. The passive power factor correction circuits usually contain a combination of large capacitors, inductors and rectifiers that operate at the frequency of the AC power line to provide a resonant circuit which produces a sinusoidal current waveform. While passive power factor correction circuits can produce a high power factor, they are not very efficient. Active power factor correction circuits, on the other hand, incorporate some form of high frequency switching converter for power processing of the voltage and current waveforms. They typically utilize microchip technology to control operation of the power supply circuit and produce a current waveform with a sinusoidal shape. A popular topology is the "boost converter" which will be familiar to those skilled the art. Since active power factor circuits operate at a frequency which is much higher than the AC power line, the circuits can be smaller, lighter in weight, and more efficient than a passive circuit.
While known power factor correction circuits have provided elegant solutions to the problem of power factor control by keeping the current drawn from the AC power line sinusoidal and "in-phase" with the line voltage, known power correction circuits still possess less than ideal characteristics. One shortcoming common to most power factor correction circuits is the presence of a current path in the switch-off state which in practical terms means that the load will be "live" or connected to the full wave rectified AC voltage requiring care and caution for testing and maintenance operations, e.g. the use of isolation transformers. This problem has been addressed in the prior art by including elaborate protection circuitry for the output stage. Another problem associated with known power factor correction circuits arises from the requirement that the voltage on the bulk capacitor must be greater than the input line voltage, i.e. V.sub.CAP .gtoreq.1.2 .sqroot.2 V.sub.INPUT to provide the capability to maintain a sinusoidal wave-shape for the current. In practical terms, this makes it next to impossible to provide a compact electronic ballast system for a high voltage application, e.g. &gt;347 volts. Another shortcoming of known power factor correction systems concerns the bulk capacitor. Because there is an off-state current or charge path, the bulk capacitor must be able to handle the voltage 1.2 .sqroot.2 V.sub.INPUT, which means that the capacitor will have a fairly high value, i.e. in micro-Farad range. Capacitors in this range are typically large in size and comprise electrolytic dielectrics. The large size of the capacitor limits the output capacity of the power supply and also makes it difficult to miniaturize the power circuit, and therefore applications such as laptop and notebook computers and line powered personal digital assistants (PDA's) are limited. Furthermore, it is not desirable to use electrolytic capacitors because of their unreliable nature as will be understood by those skilled in the art. For example, electrolytic capacitors, and specifically the dielectric layer, are prone to breakdown over time and susceptible to environmental factors such as humidity and heat.
The maximum performance for power supplies using existing power factor correction circuitry (i.e. electrolytic capacitors) is typically 30 Watts/in.sup.3. This makes it impractical to use such power supplies, e.g. switching power supplies, for applications such as laptop computers, or other electronic devices where size and weight are important.
Furthermore, the maximum power levels for known power factor correction circuits are typically in the range 400 Watts, which as will be understood by one skilled in the art makes these circuits unsuitable for high voltage or high power applications, for example, electronic ballast supplies for High Intensity Discharge ("HID") lamps.
Electronic ballast circuits are power supplies which are designed for fluorescent, high intensity discharge, halogen, etc. lighting systems. The application of known power factor correction circuits, e.g. boost converters, to electronic ballast circuits is limited because of the requirements for a high output voltage or "start voltage" to initiate the discharge and then a low output voltage or "running voltage" to maintain the discharge.
Accordingly, there is a need for power factor correction circuits which overcome the shortcomings of PFC circuits known in the art.